Membrane filters with pore sizes much larger than bacteria, 1,2,3,5,8,10, and 14 micra diameter, will effectively remove those bacteria from filtered suspensions. The phenomenon depends on surface charge, instrinsic to membrane filters and probably a by-product of the manufacturing process. Membrane filters of any material possess the charged surfaces, but cellulose and cellulose acetate polymers have more than polycarbonate filters, and thereby are more attractive to bacteria. Gram-positive organisms are attracted more than Gram-negative. Nylon, Teflon, and vinal membrane filters are far less "magnetic". Bacteria continue to attach to a membrane until a dense monolayer of particles is formed. For a 47 mm diameter cellulosic filter, roughly 1 x 10 bacterial cells are required to "saturate" the filter on continued filtration. Then a much larger percent of the bacteria pass through the filtrate, but never 100% and eventually they pile up on the filter sufficiently to slow filtration by blocking the pores. Tween 20 (0.05%), a nonionic detergent, elutes at least half of the particles with one 50 ml volume, and all particles upon repeated elution.